I can't catch any fish, because of those jellyfish. Queek.Please catch those jellyfish. Queek.If it's difficult to catch the ones underwater, use a Matchlock. Queek.Jellyfish live in lakes.※ In order to complete the quest,you'll need to defeat a jellyfish using a practice matchlock.

Tried using my matchlock and killing manually. Nothing. Did a search and apparently the kills only count if you rent the Practice Matchlock from Merio(the quest giver). Confirmed that you have to use the rented Practice Matchlock from Merio to complete this quest.